A variety of systems have been proposed for reading video record discs to derive an electric signal that represents program information stored on the disc and is suitable for application to the antenna terminals of a color television receiver. Supplying that signal to the receiver permits image reproduction of the program carried by the disc. One such system is referred to as the electrostatic or capacitive type. It differs from mechanical and optical video disc playback systems in the requirements imposed on the disc and in the nature of the pickup structure.
More specifically, a video disc for use in the capacitive system has program information recorded in known fashion in a spiral track having undulations that constitute a spatial representation of a frequency modulated carrier signal conveying the program information and under the control of which the storage track shall have been formed. The storage track has a conductive layer and a superimposed dielectric layer to the end that a conductive stylus, traveling in and sensing the record track, constitutes in conjunction with the conductive and dielectric layers of the disc, a capacitor having a value of capacitance that varies with the undulations of the record track. This capacitance variation may be employed in known manner to control the operating frequency of an oscillator, for example, to develop a frequency modulated signal suitable for control of a color receiver. As thus far described, the capacitive system is well known and, as such, is not the contribution of the subject invention.
Practical forms of the capacitive system feature a stylus of a material that is very hard compared with the material of the disc, usually taking the form of a metallized sapphire or diamond. Since, as described above, the stylus is in physical contact with the record groove and must be in order to obtain maximum signal to noise ratio, it is subject to wear. The wear characteristics of the stylus are clearly influenced by the tracking force with which the stylus is brought to bear against the track as well as the distance traveled, that is to say, the product of the linear velocity of the disc and the tracking time. It is also influenced by the coefficient of friction of the stylus. For the most part, there is little leeway in minimizing the tracking force, disc speed or program time in order to increase the life of the stylus and consequently resort has been had to the use of gem stones or lubrication in order to have as great a stylus life as possible.
Stylii have been made of sapphires and diamonds, as stated, but this is prohibitively expensive unless some means, such as that proposed herein, is found for obtaining an acceptably long stylus life. It has been proposed, for example, that the disc have a lubricated surface so that the abrasion is reduced and the life of the stylus increased. There is a distinct preference, however, to avoid lubrication since such coatings have a tendency to collect dirt, they tend also to dry out and lose their lubricating qualities especially if they are water based and may also present undesirable packaging problems.
It is, therefore, a principal object of the invention to provide a pickup structure featuring a stylus for tracking grooves in a record disc and having a useful life which greatly exceeds that of prior devices.
It is a very particular object of the invention to provide an improved pickup device featuring greatly enhanced stylus life and especially suited for reading video record discs in capacitive video playback systems.